1. Field of Invention
The present general inventive concept relates to the preparation and use of thermoelectric materials and more particularly to methods and processes to fabricate doped thermoelectric generators, especially doped silicon-based thermoelectric generators.
2. Description of the Related Art
Semiconductive materials that exhibit the Seebeck and Peltier effects in the presence of a temperature gradient are useful for the production of electricity from waste heat. Semiconductive materials which move heat from one side to the other when presented with an electrical charge from one side to the other are useful for cooling and exhibit the Seebeck effect in the form of the Peltier phenomena. The class of semiconductive materials exhibiting the Seebeck and Peltier effect is hereinafter called thermoelectrics or thermoelectric materials.
A number of contemporary thermoelectrics comprise alternating P-type and N-type semiconductor elements connected by metallic connectors. Many contemporary thermoelectrics present various disadvantages, including, in some instances, high material costs, high costs of production, difficulty of manufacture, the use of rare elements, the use of potentially carcinogenic or toxic substances, and limited formability.
To achieve a high level of conversion, both a high thermoelectric figure-of-merit (ZT) and a high operating temperature are required.
The Seebeck Coefficient (S) of a material is a measurement of the magnitude of an induced thermoelectric voltage in response to a temperature difference across that material. Optimally, a highly efficient thermoelectric material should have a high Seebeck Coefficient, high electrical conductivity, and low thermal conductivity and be able to operate at high temperatures, meaning it should have a low coefficient of thermal expansion. See, e.g., Ci et al., Materials Letters 65, 1618-1620 (2011). Other considerations arise as well. For instance in order to sustain a high temperature difference from one side to the other a low coefficient of thermal expansion, low Poisson ratio and high strength are desirable. It is desirable that a thermoelectric material be susceptible to being worked to construct planar and complex net-shaped objects that can be fitted into locations where they may be used to recover waste heat. Such a thermoelectric material should have a cross section with properties to maintain a sufficiently high temperature differential between the two opposing sides in order to generate voltage efficiently. It is also desirable that a thermoelectric material have high tensile strength, have resistance to thermal shock, and be formable into layers to allow the creation of graded indices for electrical, thermal, or other parameters—allowing one thermoelectric material to serve as the basis for a range of thermoelectric devices.
The thermoelectric figure-of-merit, ZT, for a thermoelectric material (TEMat) is a measure of its efficiency. Z is calculated by multiplying electrical conductivity (s) and Seebeck Coefficient (S) squared and dividing by thermal conductivity (k), or Z=S2σ/k, and ZT is calculated by multiplying Z with absolute temperature (in Kelvin). To achieve a high power factor, it is therefore desirable to have a TEMat with low thermal conductivity, high electrical conductivity, high Seebeck Coefficient, and with a high temperature operating capability (i.e., a sustainable temperature difference across its structure or DT capability).
But potentially exploiting a TEMat's ZT is more than materials science. Successful exploitation will need to combine brittle material engineering practices as TEMats, as a material class, are very brittle (i.e., low fracture toughness). A prerequisite to exploiting a TEMat high temperature capability and its ZT is it must be able to also mechanically withstand a large DT in service. This in turn results in a need for the TEMat to have a minimum coefficient of thermal expansion (CTE) and maximum tensile strength (STen). Lastly, from a perspective of size, a larger TEMat component or “leg” will promote the ability to achieve a larger DT (presuming it does not mechanically fail); this is an important issue for achieving cold temperatures too.
Incumbent technologies offer little hope of making low cost thick structures able to operate at high temperatures with high unaided DT and attractive power factors. Traditional and new approaches to making thermoelectric generators (TEGs) are all flawed by fundamental and seemingly intractable challenges, such as high cost, high CTE, limited to thin planer structures, low S, low electrical conductivity, low mechanical strength, or use of rare and costly materials, or combinations thereof. Many of those same issues limit the ability to achieve colder temperatures with thermoelectric coolers (TECs).
Also, thermoelectric materials, as a material class, are very brittle. Therefore, it is also desirable to be able to fashion a thermoelectric material with reduced brittleness.
An ideal pathway for making thermoelectric devices would include a way to obtain nano sized equiaxed silicon grains that could be formed into robust large shapes with large cross sections and a nano structured morphology, so to achieve or promote a low CTE, a low value for k, very high values for s, gain high S values, and high operating temperature capability.
Many of the recent efforts and developments in this field have focused on nanowires and MEMS, which have brought forward announcements confirming exceptionally high power factors with very high efficiencies in converting waste heat to electricity. Unfortunately, these structures are expensive and cannot be practically made in the thick cross sections required to maintain a large unaided or largely unaided ΔT. Many of the results reported used aggressive heat exchange apparatus to maintain a high ΔT. In many or most cases, these aggressive heat exchange apparatus are also necessary to limit the ΔT in order to avoid catastrophic thermomechanical failure of the thermoelectric materials.
Some thermoelectric generators employ compounds and elements such as tellurium or rare earth metals—many of which are scarce, sourced from only a few locations. For operators working in North America, many such materials must be imported (for example, most rare earth metals at this time are imported from China). It is desirable to have a thermoelectric material that does not require tellurium, rare earth metals, and similarly rare component materials.
Wang et al. (“Effect of Grain Sizes and Shapes on Phonon Thermal Conductivity of Bulk Thermo Electric Materials,” Journal of Applied Physics 110, 024312 [2011]) teach that silicon's thermal conductivity is insensitive to grain size until the grain sizes are reduced to quite a bit less than a micron, and then falls precipitously from about 600 nm to 5 nm with thermal conductivity falling to less than 0.4 W/mK. But they only address a “bulk” material. They do not describe methods or sources for a silicon bulk material with a grain size in the range of a few or tens of nanometers, but conclude that only by reducing the grain size can one obtain silicon with very low thermal conductivity.
U.S. Pat. No. 8,334,194, issued to Jonczyk and Rand, discloses methods and apparatus for fabricating a semiconductor sheet. In one aspect, a method for fabricating a semiconductor wafer includes applying a layer of semiconductor material across a portion of a setter material, introducing the setter material and the semiconductor material to a predetermined thermal gradient to form a melt, wherein the thermal gradient includes a predetermined nucleation and growth region, and forming at least one local cold spot in the nucleation and growth region to facilitate inducing crystal nucleation at the at least one desired location.
U.S. Pat. No. 9,011,763, issued to Chen et al., discloses nanocomposite thermoelectric materials that exhibit enhanced thermoelectric properties. The nanocomposite materials include two or more components, with at least one of the components forming nano-sized structures within the composite material. The components are chosen such that thermal conductivity of the composite is decreased without substantially diminishing the composite's electrical conductivity. Suitable component materials exhibit similar electronic band structures. For example, a band-edge gap between at least one of a conduction band or a valence band of one component material and a corresponding band of the other component material at interfaces between the components can be less than about 5kBT, wherein kB is the Boltzman constant and T is an average temperature of said nanocomposite composition.